ifupdownmain: ifquery check read_iface_config errors

ifupdownmain: ifquery check read_iface_config errors

auto swp1
iface swp1
      typo_attribute 42

$ ifquery -ac
error: /etc/network/interfaces: line9: iface swp1: unsupported keyword (typo_attribute)
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp                                                [pass]

auto swp1
iface swp1

$ echo $?
1
$
$ echo $?
1
$ ifup -a
error: /etc/network/interfaces: line9: iface swp1: unsupported keyword (typo_attribute)
$ echo $?
1
$

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>